When Milky Way and Andromeda Collide, Earth Could Find Itself Far From Home

NASA/HUBBLE HERITAGE TEAM (STScI)
If Homo sapiens can stick it out on Earth for another two billion years, our descendants may witness quite a show in the night sky. Researchers estimate that the Milky Way will collide with its nearest neighbor, the Andromeda galaxy, at around that time—well before the sun collapses into a white dwarf, perhaps destroying the Earth in the process.

This close encounter of the galactic kind could easily kick our solar system to the farthest reaches of the galaxy, and there is a small chance we might even take up residence in Andromeda, according to astronomers T. J. Cox and Abraham Loeb of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.

The pair simulated the collision by estimating the relative speed between the two galaxies and the amount of gas and dark matter in the intervening space, which exerts a drag on their motions.

Andromeda is currently 2.3 million light-years from our galaxy. Researchers know that the two neighbors are approaching each other at 120 kilometers per second, but they are far less certain of Andromeda's sideways speed. If moving fast enough to the side, it would miss us entirely.

"I think it's very likely they will come together," Loeb says. "The issue is, will it be [in] three billion years, five billion years or 10 billion years?"

Taking their cue from the latest models of the galaxies' structures, Cox and Loeb assumed a relatively small sideways motion. Based on this assumption, Andromeda would first graze the Milky Way two billion years from now, they report in a paper submitted to the Monthly Notices of the Royal Astronomical Society. The two galactic cores would orbit each other for another three billion years before merging.

During that time, the stars making up the two spiral galaxies would slowly coalesce into a more elliptical combo galaxy, "Milkomeda" (or the Andromedy Way, if you prefer). Although most of the stars would be too sparsely spaced to bump together, one galaxy's gravity would jostle the other's stars.

The fate of the sun, which is expected to last at least until the simulated merger, would depend on where it was in its 24,000 light-year-wide orbit around the galactic core. The researchers estimate that by the time the cores had fused, the solar system would have a 50 percent chance of being swept to a wispy tail extending from Milkomeda, three times further out from galactic center than it is now.

Cox and Loeb also find a 3 percent chance of the sun being nudged into orbit around Andromeda when the two galaxies first collide. Of course, they note, different assumptions for the simulation would likely result in different outcomes.

"What's cool about this," says astronomer Gregory Laughlin of the University of California in Santa Cruz, "is they track a reasonable orbit for the sun and sort of give a plausible range of scenarios for what the solar system might encounter. It's fun to speculate on this stuff."